Implantable heart valve prostheses have been used to replace various diseased or damaged native or natural aortic valves, mitral (bicuspic) valves, pulmonic valves and tricuspid valves of the heart. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left heart chambers responsible for maintaining cardiac output of oxygenated blood. The aortic and mitral valves are most frequently replaced due to heart disease, congenital defects or injury. The mitral valve controls the flow of blood between the left atrium and the left ventricle and the aortic valve controls the blood flow from the left ventricle into the aorta. Generally, the known heart valve prostheses are either bioprostheses or mechanical heart valve prostheses.
Modern mechanical heart valve prostheses are typically formed of an annular valve seat in a relatively rigid valve body and one or more occluding disk or pair of leaflets that is movable between a closed, seated position in the annular valve seat and an open position in a prescribed range of motion. Such mechanical heart valves are formed of blood compatible, non-thrombogenic materials, typically currently comprising pyrolytic carbon and titanium. Hinge mechanisms and/or pivoting guides entrap and prescribe the range of motion of the disk or leaflets between open and closed positions. Exemplary bi-leaflet mechanical heart valves are disclosed in commonly assigned U.S. Pat. Nos. 4,935,030 and 6,139,575 and in U.S. Pat. Nos. 6,176,877 and 6,217,611.
The bioprostheses or “tissue valves” or “xenografts” (hereafter “tissue valves”) are generally made of suitable donor heart valves harvested from an animal and treated with a preservative and fabricated as described further herein. The most widely used tissue valves include some form of stationary metal or plastic frame or synthetic support, referred to as a “stent”, although so-called “stentless” tissue valves are available. The most common tissue valves are constructed using an intact, multi-leaflet, harvested donor tissue valve, or using separate leaflets cut from bovine (cow) pericardium, for example. The most common intact donor tissue valve used for stented and stentless valves is the porcine (pig) aortic valve, although valves from other animals (e.g., equine or marsupial donors) have been used. The present invention is not limited to the preparation of porcine valves, though existing tissue valves on the market are nearly exclusively made from porcine valves, and thus the description herein will focus on such tissue valves.
Exemplary tissue valves formed of swine valve leaflets mounted to struts of a stent are those disclosed in U.S. Pat. Nos. 4,680,031, 4,892,541, and 5,032,128 as well as the MEDTRONIC® Hancock II® and Mosaic® stented tissue valves. Some prosthetic tissue valves are formed from treated integral swine valve leaflets and valve annulus structure, e.g., the MEDTRONIC® Freestyle® stentless aortic root bioprostheses.
Mechanical and tissue valves have advantages and disadvantages. By their very nature, mechanical heart valves have metal or plastic surfaces exposed to the blood flow, which remain thrombogenic even long time after their implantation by major surgery. The opening and closing of mechanical heart valve occluders can damage blood elements and trigger a coagulant cascade. Blood flow disturbances in certain mechanical valves are also believed to aggravate blood coagulation. Therefore, patients having such mechanical heart valves can avoid potentially life threatening embolus formation only by taking anti-thrombogenic or anti-coagulant medication on a regular basis. Porcine tissue valves include three cusps or leaflets of a heart valve excised from pigs and preserved by treatment with glutaraldehyde. The preserved porcine tissue is thrombogenic, and therefore, the human patient takes anti-thrombogenic or anti-coagulant medication at least a period of time after the surgical implantation of a tissue valve. Valve leaflet opening and closing characteristics and blood flow past open tissue leaflets of tissue valves can be superior to those afforded by mechanical valves. However, tissue leaflets can become calcified over time distorting the leaflet shape and ultimately leading to failure of the tissue leaflets to fully close or open. Proposals have been advanced to form mechanical heart valve prostheses from flexible, anti-thrombogenic, polymeric sheets or fabrics that are resistant to calcification mounted to stents to function like stented tissue valves also been proposed as exemplified by U.S. Pat. No. 5,562,729. However, calcification and tear issues of polymeric materials remain to be solved before a polymeric valve can be realized.
Such mechanical valves and tissue valves are intended to be sutured to peripheral tissue of a natural heart valve orifice (the “valvar rim”) after surgical removal of damaged or diseased natural valve structure from the patient's heart. Modern prosthetic heart valves are typically supplied with a sewing or suturing ring surrounding the valve body or stent that is to be sutured by the surgeon to the valvar rim. Suturing rings typically comprise a fabric strip made of synthetic fiber that is biologically inert and does not deteriorate over time in the body, such as polytetrafluoroethylene (e.g., “Teflon PTFE”) or polyester (e.g., “Dacron”), that is woven having interstices permeable to tissue ingrowth. The valve body or stent is typically circular or ring shaped having an outer surface or sidewall shaped to fit with an inner sidewall of the suturing ring. In some cases, the suturing ring fabric is shaped to extend outward to provide a flattened collar or skirt that can be applied against and sutured to the valvar rim, as shown for example in U.S. Pat. No. 3,997,923.
To assure a proper fit, the patient's tissue annulus must be “sized” to indicate the size of the mechanical valve or tissue valve to be implanted in the valvar rim. In particular, proper fit of the annular valve body within the tissue annulus of the excised native valve is required. Typically, a set of sizers is supplied by the prosthetic heart valve manufacturer corresponding to the different sizes of available prosthetic heart valves. The surgeon inserts the sizers through the prepared annulus defined by the valvar rim to determine which corresponding prosthetic heart valve will best fit the valvar rim. The surgeon usually must attempt to size the annulus several times with one or more sizers of different diameters until the best fit is recognized.
Conventional sizers for measuring the human valve annulus typically comprise a series of incrementally sized cylindrical elements marked with the corresponding outside diameter in mm. Most sizer sets include cylindrical elements that range from a low of 19 mm to a high of 33 mm, in 2 mm increments, and a common handle for manipulating the sizers. Some sizers for measuring the human valve annulus are shaped, or include flanges or other stepped features to also provide a measurement of the aortic root adjacent to the annulus. The aortic root is that part of the valve anatomy between the annulus and the convex sinuses of the ascending aorta, and has a generally scalloped appearance with the valve leaflets being attached along alternating arcuate cusps and upstanding commissures around its border. In any event, the primary measurement derived from conventional surgical sizers is the annulus diameter determined by finding which sizer fits properly in the annulus based on tactile feedback.
A different approach to sizing the prepared valvar rim annulus is set forth in U.S. Pat. No. 6,110,200. The '200 patent discloses a single sizing apparatus that can be adjusted through a range of diameters to measure an anatomical tissue annulus. The apparatus includes an elongated support member having a proximal end and a distal end. An operator actuated movable member is joined to the proximal end of the elongated support member while an adjustable member is joined to the distal end of the elongated support member. The adjustable member has a reference axis and an outer curved surface selectively positionable in response to the operator actuated member. In particular, the outer curved surface can be selectively positioned between an inner position proximate the reference axis and an outer position spaced apart from the reference axis.
In one embodiment, a fluid passageway extends from the proximal end to the distal end an elongated support member, and an operator actuated movable member is joined to the proximal end of the elongated support member for providing fluid through the passageway to the distal end. An expandable, balloon-like member mounted to the distal end of the elongated support member receives the fluid. The expandable member having a reference axis and a substantially continuous outer curved surface disposed about the reference axis and selectively positionable in response to fluid provided by the operator actuated movable member between an inner position proximate the reference axis and an outer position spaced apart from the reference axis.
Preferably, an indicator is mounted to the elongated support member for determining a dimension of the outer curved surface. The indicator includes indicia representative of selected radial distances of the outer curved surface from the reference axis. A detent mechanism rotates with the shaft and selectively engages releasable stop surfaces on the indicator. The stop surfaces correspond to the various indicia on the indicator. The stop surfaces hold the shaft in each of the selected angular positions, which in turn, maintains the outer curved surface at each selected radial distance from the reference axis.
Thus, it is necessary to manufacture and distribute prosthetic heart valves in a variety of sizes to fit the sized annulus of a prepared valvar rim. The tissue valve manufacturer must have a supply of fresh donor heart valves from animals from which the range of tissue valves are fabricated. A typical prosthetic valve fabrication process for manufacturing porcine tissue valves in a variety of sizes is described in U.S. Pat. No. 6,458,155, for example.
As set forth in the '155 patent, fresh porcine hearts are first harvested in a certified slaughterhouse, weighed, and sorted into various valve size ranges by either estimating the annulus by eye, based on the flattened aortic width, or by estimating valve orifice size from heart weight. Of course, this correlation is a very rough estimate, with actual valve annulus sizes differing quite a bit within similarly sized porcine hearts. The aortic valve and surrounding tissue including a section of the pulmonary artery (hereinafter termed the “aortic valve isolation”) is then severed from the porcine heart.
A batch of aortic valve isolations are packed in ice and shipped from the slaughterhouse to the prosthetic valve manufacturing facility. At the manufacturing facility, the aortic valve isolation is further sorted by valve size by technicians trained to estimate such valve size using their fingers. That is, the orifice diameter of the aortic valve annulus is estimated by insertion of one or more fingers through the inflow end of the aortic valve isolation. Because of the rough nature of the heart weight to valve size estimation, a large proportion of valves are rejected at this stage, resulting in wasted inventory and shipping costs.
The accepted aortic valve isolations are then trimmed and chemically fixed to render the donor tissue valves biologically inert for implantation in human patients. The trimming procedure typically involves cutting away the pulmonary artery and surrounding muscle tissue from the inflow end of the aortic valve isolation. The remaining donor tissue valve is generally tubular having a small amount of tissue on the inflow side of the annulus and the internal leaflets enclosed and protected by the tubular ascending aorta.
Chemical fixation may be accomplished using a variety of techniques and chemicals, though the most common procedure used involves supporting the tubular tissue valve element on at least the ascending aorta or outflow portion with a fixation insert, immersing the assembly in a bath of fixing solution (e.g., glutaraldehyde), and either flowing fixing solution through the tissue valve element or maintaining a predetermined pressure differential across the leaflets during the fixation process. See, for example, U.S. Pat. No. 4,372,743, which describes maintaining a low pressure differential across the leaflets of between 1-4 mm Hg.
The use of fixation inserts is also quite effective in shaping the tissue valve during the fixation process. For example, U.S. Pat. No. 5,197,979 describes inserts having three outwardly convex regions for shaping the tissue valve sinuses. More recently, U.S. Pat. No. 6,001,126 discloses inserts having a plurality of pinholes in the two convex regions corresponding to the coronary sinuses that enable coronary artery shaping plugs or mandrels to be mounted thereon. Whichever type of insert is used, the ultimate size of the fixed tissue valve is influenced, at least in the sinus regions, by the insert. Preferably, the relative size of the annulus and sinus regions is identical to the human aortic tissue valve being replaced. It is therefore very important to begin with a donor tissue valve having an accurately sized annulus.
The fixation process causes some shrinkage in the donor tissue valve. Therefore, the sizing of the annulus of the fresh donor tissue valve provides only an estimate of the annulus size of the fixed tissue valve. The amount of shrinkage depends on the chemicals used, the duration of fixation, the pressure differentials maintained within the tissue valve annulus during fixing, the temperature during fixing, and other less significant factors. Because of these variables, the annulus of fixed porcine aortic tissue valves are sized once again using a caliper and/or a sizing stent to sort the tissue valves into mounting sizes.
It is thus apparent that an accurate and reliable means for estimating, from the fresh donor tissue valve, the annulus size of a fixed xenograft tissue valve annulus is needed to increase tissue valve yield and quality, and reduce expense.
The above-referenced '155 patent discloses an apparatus for sizing fresh donor heart tissue valves that have a lumen and an inwardly directed tissue valve annulus within the lumen. The apparatus includes a sizing member having an axially extending, preferably conical, sizing portion with a forward end adapted to insert within the lumen of the donor heart tissue valve. The sizing portion increases in size along an axis from the forward end such that a region on the exterior thereof eventually contacts the tissue valve annulus upon continued insertion of the sizing portion within the lumen. A measuring bracket connects to the sizing member and has a scale portion spaced from and generally aligned with the sizing portion, the scale portion providing markings indicating the annulus size of the donor heart tissue valve relative to the position of the donor heart tissue valve on the sizing portion. The measuring bracket may include a mounting portion generally perpendicular to the scale portion and including a through hole into which the sizing portion fits. The markings are desirably supplemented with numerical indicators of tissue valve size, either in terms of tissue valve diameter in millimeters or as non-dimensional numbers in conjunction with a separate chart to correlate the numerical indicators with tissue valve size. The markings may be calibrated for fresh donor tissue valves from a particular geographic supply source.
The method of use of the sizing member disclosed in the above-referenced '155 patent includes inserting the forward end of the sizing portion into the donor heart tissue valve lumen, and halting the insertion at a predetermined resistance to further insertion. After halting further insertion of the sizing portion into the lumen, the tissue valve annulus size is determined based on the distance that the sizing portion has been inserted. In a preferred embodiment, the sizer further includes a measuring bracket connected thereto having a scale portion spaced from and generally aligned with the sizing portion. The tissue valve size is determined by observing the position of the donor heart tissue valve with respect to the scale portion of the measuring bracket.
A number of problems can arise in using the sizing member of the '155 patent. The user must select the tissue valve isolation and insert the tip of the conical sizing member into the valve lumen, pushed the tissue valve isolation over the conical surface as far as the valve annulus will go, to read the indicia on the surface, and to then pull the tissue valve isolation back off the conical sizing member. In doing so, care must be taken to avoid stretching the tissue valve annulus unduly. It is necessary for the user to keep the conical surface clean so that the conical sizing member can be easily inserted into the tissue valve lumen to the correct extent and then be easily removed from the tissue valve lumen. The estimation of the tissue valve lumen size is somewhat subjective for these reasons and because the tissue valve isolation covers the indicia on the conical surface at the tissue valve lumen.
It would also be difficult to use the conical probe within the lumens of harvested donor tissue valve isolations that include a long, curved aortic root as is used in preparation of certain tissue valve prostheses.